This invention relates to a stencil forming apparatus which make minute holes on a stencil pulled out from a rolled stencil sheet.
FIG. 7 shows a conventional stencil forming apparatus. A stencil printing sheet S for the stencil printing (hereinafter referred to merely as "a stencil sheet S") which is in the form of a roll is held by a master holder 106. A stencil sheet S pulled out of a rolled stencil sheet S is in the form of a belt. The stencil sheet S is laid over a set guide shaft 100 to change the direction of its movement, and inserted between a thermal printing head (TPH) 101 and a platen roller 102, and then laid over a winding roller 103. Furthermore the stencil sheet S is inserted between an upper load roller 104 and a lower load roller 105, thus being conveyed to an initial stop position.
In order that, when the stencil sheet S is conveyed, the part of the sheet which is held between the thermal head 101 and the platen roller 102 may not be creased, a back tension unit 107 of the master holder 106 applies tension to the stencil sheet S in the direction which is opposite to the direction of conveyance of the stencil sheet S.
When the thermal head 101 thermally makes fine holes on the stencil sheet S, the power of conveying the stencil sheet S is provided by the platen roller 102 only which holds the stencil sheet S with the thermal head 101. The stencil sheet S thus formed is moved downwardly into a holding box 108 by means of the winding roller 103. Until the thermal cutting of the stencil sheet S is accomplished by the thermal head 101, the stencil sheet S thus cut is temporarily held (stored) in the holding box 108.
After the stencil sheet S has been formed with the thermal head 101, the latter 101 is moved upwardly to release the stencil sheet S. The front end portion of the stencil sheet S thus released is conveyed to a clamp board 110 on a printing drum 109 by means of the upper and lower load rollers 104, so that the front end portion of the stencil sheet S is secured to a printing drum 109 with the clamp board 110. As the printing drum 109 turns, the upper and lower load rollers 104 and 105 are turned to convey the stencil sheet S so that the sheet S is wound on the printing drum. Thereafter, the stencil sheet S is cut off with a cutter unit 111.
The above-described stencil forming apparatus serves also as a stencil printing machine. Part of the structure of the stencil printing machine is shown in the drawings. A part of the cylindrical wall of the printing drum 109 is ink-permeable. An ink supplying means for supplying ink to the inner surface of the printing drum 109 is provided inside the drum 109. A pressing means, namely, a press roller is provided below the printing drum to press a printing sheet against the printing drum 109. The printing drum 109 is turned, while a printing sheet is fed into the space between the printing drum 109 and the pressing means, so that the printing sheet is pressed against the image region of the stencil sheet S wound on the printing drum 109. The printing ink supplied from the inner cylindrical surface of the printing drum 109 passes through the cylindrical wall of the printing drum 109, and transfers through the cuts of the stencil sheet S onto the printing sheet to form an image thereon.
The conventional stencil forming apparatus shown in FIG. 7 suffers from the following difficulties: Because the set guide shaft 100 and the platen roller 102 are not in parallel with each other, the stencil sheet S being conveyed is liable to slacken, and, when the stencil sheet S is cut, it is liable to be creased.